Production of alkali metal borohydrides



United States Patent PRODUCTION or ALKALI METAL BOROHYDRIDES William H. Schechter, Zelienople, Pa., assignor to Qallery Chemical Company, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Original application March 17, 1953, Serial No. 342,993. Divided and this apphcatlon November 21, 1956, Serial No. 623,539

4 Claims. (CI. 23-14) This invention relates to the production of alkali metal borohydrides, MBH in which M is an alkali metal, and more particularly to their production by the reaction of an alkali metal, hydrogen, and an alkali metalfluoborate, MBF

The metallic borohydrides are excellent reducing agents, they provide efiicient sources of hydrogen, and they may be used for the production of various boron compounds, e.g., diborane and related boranes, which in turn are desired for certain purposes, e.g., for thermal conversion to other boron hydrides. The methods known for the production of boranes and metallic borohydrides are characterized by low yields or they require highly skilled operating technique. Generally speaking, they are adapted only to batch operation, and the products made by procedures known heretofore are consequently quite ex-.

pensive.

The primary object of the invention is to provide a simple and easily performed method for the making of alkali metal borohydrides which is simple and efiicient, may be practiced with readily available inexpensive starting materials, is not dependent upon highly skilled operating technique, and is more direct and more easily practiced to provide these products at less expense than is the case with previously known methods.

A further object is to provide a method embodying the foregoing object which is adapted to continuous operation.

Another object is to provide metallic borohydrides in a form exhibiting low bulk density and of exceptionally high activity.

A further object is to provide sodium borohydride in accordance with the foregoing objects.

Other objects will be recognized from the following specification.

This invention is based on the discovery that a finely divided alkali metal, hydrogen, and an alkali metal fluoborate react at elevated temperatures according to the equation 4M+2H +MBF =MBH +4MF to produce an alkali metal borohydride. The reaction is performed in a closed system and under anhydrous conditions. Alkali metals, such as sodium, potassium or lithium, or mixtures of alkali metals with one another such as sodium-potaspotassium alloy, or with other metals may be used. Also, any alkali metal fluoborates, such as lithium, sodium or potassium fluoborate may be used in the foregoing reaction with corresponding production of alkali metal borohydride.

In the preferred embodiment of the invention the alkali metal is dispersed in an inert liquid. The alkali metal is dispersed as finely as possible in the heated inert liquid,

'most suitably one of high boiling point such as mineral oil. Other appropriate inert liquids are, of course, available and will occur to workers in the field. Hydrogen and the alkali metal fiuoborate are then introduced into the heated alkali metal suspension, or dispersion, while agitating or stirring it violently. If preferred, the alkali metal fluoborate may be introduced along with the alkali "ice metal. Maintenance of fine dispersion during the reaction is, I find, essential. This may be accomplished suitably by the use of a high speed stirrer, such as a flat bladed turbine or a slotted impellor of the type known as a Dispersato 1'. Hydrogen may be provided for reaction simply by maintaining a hydrogen atmosphere over the agitated dispersion, or it may be injected into the agitated dispersion, or it may be injected through the agitator bearing unit so as to be expelled through perforations in the stirrer blades. It is preferred to use bafiled containers in accordance with modern mixing practices.

In the production of alkali metal borohydrides according to the foregoing reaction the alkali metal, and the alkali metal fluoborate, finely dispersed or suspended in an inert liquid is heated to 250 to 265, or higher, e.g. 320 C., and while agitating violently, dry hydrogen is introduced into this suspension. When reaction has been completed the reaction body is cooled and filtered, and the solid residue is washed with solvent for the suspension liquid, e.g. petroleum ether in the case of mineral oil. The borohydride is then extracted from the alkali metal halides, as by liquid ammonia at 33 C. and the bor0- hydride then recovered by evaporation of the ammonia. Other selective solvents for the borohydride, such as amines and polyethylenedialkylethers, may also be used.

Following this procedure sodium borohydride of as high as to 94% purity has been made using sodium fluoborate and sodium as the alkali metal from reactions of 4 hours duration at 280 C. I

At temperatures below 250 C., e.g. 220 C., the reaction is so slow as to be impractical. It has been found that at higher temperatures, i.e. at about 320, that secondary reactions of an undetermined character decrease the yield of borohydride. A reaction of 3 hours duration at 275 C. gave a 75% yield of sodium borohydride, while a reaction of 3 hours duration at 320 C., resulted in a yield of only 27%. Although the desired reaction proceeds to some extent at higher temperatures, i.e. 350 C., it is preferred to operate at temperatures below 320 C. The reaction is exothermic, and the inert liquid functions as a heat sink and increases the rate at which heat can be transferred to and from the reaction, thus facilitating the temperture control of the reaction.

In the practice just described the practice may be operated at or slightly above atmospheric pressure. I find, however, that it is advantageous to operate in an autoclave under superatmospheric, e.g. at or p.s.i.g., as at elevated hydrogen pressures the rate at which the reaction proceeds is increased.

The reaction may be aided, if desired, by having present in the suspension a small amount of stearic acid as a catalyst.

This application is a division of my copending application, Serial Number 342,993, filed March 17, 1953, which is a continuation-in-part of my application, Ser. No. 229,142, filed May 31, 1951, both now abandoned.

According to the provisions of the patent statutes, I have explained the principle and mode of practicing my invention and have described what I now consider to represent its best embodiment. However, I desire to have it understood, that within the scope of the appended claims, the invention may be practiced otherwise than as specifically, described.

I claim:

1. A method of making an alkali metal borohydride comprising dispersing an alkali metal in an inert liquid at a temperature of about 250 to 350 C., and introducing dry hydrogen and an alkali metal fluoborate into the dispersion, and thereby producing said borohydride, and recovering said borohydride from the system.

2. A method according to claim 1 in which the alkali metal is'sbdium, the alkali metal fluoborate is sodium fluoborate and the borohydri'de recovered is sodium bor'ohydride. V 2. 2

3. A method according to claim 2 in whichihe. inert liquid is mineral oil. 2 j 4. 'A method according to claim 3 in which the pressure is'bctweenatmospheric and 150 p.s.i. 2

References Cited in the file of this patent UNITED STAIES PATENTS 2,469,879 Hurd May 10, 1949 2,532,217 Winternitz N0,v.,28, 1950 2,720,444 Banus -2. Oct. 11,1955

Banug App '10, 1956 

1. A METHOD OF MAKING AN ALKALI METAL BOROHYDRIDE COMPRISING DISPERSING AN ALKALI METAL IN AN INERT LIQUID AT A TEMPERATURE OF ABOUT 250*C., AND INTRODUCING DRY HYDROGEN AND AN ALKALI METAL FLUOBORATE INTO THE DISPERSION, AND THEREBY PRODUCING SAID BROHYDRIDE, AND RECOVERING SAID BOROHYDRIDE FROM THE SYSTEM. 